As a child my favorite books(and thus the first ones I remember loving) were:
Harold and his Purple Crayon
The Monster at the End of the Book
and The Giving Tree (which admittedly has a much different meaning to me now)

The first “grown up” book I remember reading and loving (at ~11) was Watchers by Dean R. Koontz

My favorite books as an adult are:
Sirens of Titan by Kurt Vonnegut
and The Watermelon King by Daniel Wallace